Ok, I found a little more on this subject as I posted a little more on GameFAQS.
From what I am told, this is all you have to do.
Steps:
1) Have the pokemon you want to dupe in your GBA.
2) The pokemon in colosseum will become the pokemon in the GBA so make sure its something you don't want like a weak guy from the earlier parts of town in Ruby/Sapphire/Fire Red/Leaf Green.
3) Begin the trade from colosseum.
4) When its time to save it will take about 6 to 7 seconds.
5) Right before it finishes saving you have to turn off the GBA.
6) It should say on the TV "Trade Successful" or something along those lines.
7) When you turn on your GBA it will say "Save Disrupted" or something like that and it will revert to the previous save on there. You "should" have the duplicated the pokemon, having it both in GBA and Colosseum.
Can anyone confirm this, I'd like to but I can't seem to get the timing right.
EDIT: *When I say trade in colosseum, I hope you all know I mean to go to Phenac city to do so.*